Re: Threading model for reading 1,000 files quickly?

Robert Klemme <>
Wed, 03 Oct 2012 13:58:53 +0200
On 03.10.2012 09:24, Chris Uppal wrote:

I must admit that I had forgotten that aspect of the situation.

To me it seems there are a lot more "forgotten aspects"...

Consider: would you choose the time when you've got a big disk operation
running (copying a huge number of files say) to kick off a virus scan on the
same spindle ? I most certainly would not, perhaps your experience has been

File copying is only IO with negligible CPU, virus scanning only looks
at portions of files. We do not know whether that scenario only
remotely resembles the problem the OP is trying to tackle.

The problem is that the analysis in terms of scattered disk blocks is
unrealistic.If the blocks of each file are actually randomised across the
disk, then the analysis works. But in that case a simple defrag seems to make
more sense to me.

Not all file systems support online or offline defragmentation and we do
not even yet know the file system. Heck, files may actually reside on
some type of network share or on a RAID storage with it's own caching
and read strategies. Also, since defragmentation usually works on a
whole file system the cost overhead might not pay off at all. Btw.
another fact we do not know yet (as far as I can see) is whether this is
a one off thing or the processing should be done repeatedly (in case of
one off the whole discussion is superfluous as it costs more time than
the overhead of a sub optimal IO and threading strategy). It may also
make sense to know how files get there (maybe it's even more efficient
to fetch files in Java with a HTTP client from where they are taken and
process them while downloading, i.e. without ever writing them to disk).

 If, on the other hand, the block/s/ in most files are
mostly contiguous, and each thread is processing those blocks mostly
sequentially, then running even two threads will turn the fast sequential
access pattern

     B+0, B+1, B+2, ... B+n, C+0, C+1, C+2, ... C+m

into something more like:

     B+0, C+0, B+1, C+1, ... B+n, C+m

which is a disaster.

We cannot know. First of all we do not know the size of files, do we?
So files might actually take up just one block. Then, the operating
system might actually be prefetching blocks of individual files when it
detects the access pattern (reading in one go from head to tail) to fill
the cache even before blocks are accessed.

Oh, and btw., we do not even know the read pattern, do we? Are files
read from beginning to end? Are they accessed more in a random access
fashion? And we do not know the nature of the processing either. At
the moment we just know that it takes one to two seconds (on what
hardware and OS?) - but we do not know whether that is because of CPU
load or IO load etc.

Of course, my analysis also depends on assumptions about the actual files and
their layout, but I don't think the assumptions are unreasonable. In fact, in
the absence of more specific data, I'd call 'em good ;-)

That's a bold statement. You call an analysis "good" which just fills
in unmentioned assumptions for missing facts - a lot of missing facts.



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